The present invention relates to an X-ray based measuring device, and more particularly to a technique which can be suitably applied to an X-ray measuring device having a planar X-ray detector.
In a conventional X-ray based measuring device, after an X-ray image transmitted through an inspection object is first converted into an optical image by an X-ray II (X-ray image intensifier), the optical image is converted into an analog image signal. The analog image signal is converted into a digital image signal (image data) by an A/D converter, is then subjected to known image processing such as filtering and contour enhancement by an image processing circuit, and is displayed on an display as an X-ray image. In recent years, there has been a strong demand for rendering the X-ray based measuring device compact, and attempts have been made to render the device compact through the improvement of an X-ray source and its power supply. Meanwhile, in order to pick up an X-ray image of a large area in one imaging, an imaging system including the X-ray II and a television camera has tended to be large in size, and the X-ray II, in particular, has constituted a large hindrance to the miniaturization of the device because of its basic principle. As a means for overcoming this problem, the development of planar X-ray detectors has been undertaken energetically in recent years, and a method has been developed in which an X-ray image is converted into visible light (optical image) by a scintillator such as a CsI, and after the visible light is converted into electrical signals by photodiodes formed of such as a-Si (amorphous silicon), the electrical signals are sequentially read by a TFT (thin film transistor) or CMOS (complementary) circuit. In a case where a reading circuit is formed by TFTs, the planar X-ray detector makes it possible to make the screen large. On the other hand, in a case where the reading circuit is formed by a CMOS circuit, a planar X-ray detector of a large screen can be constructed by combining a plurality of CMOS X-ray detectors.
As a result of studying the conventional art, the present inventors found out the following problems. Conventional measuring devices using planar X-ray detectors have suffered from problems in that the dynamic range is narrow and that the imaging speed is slow. FIGS. 25, 26, 27, and 28 are diagrams explaining a schematic configuration of an example of a conventional X-ray based measuring device. In a case where, in chest roentgenography of a subject, an interested area C (901c) including both the heart and the lung filed and indicated by slanting lines is imaged as an interested area, as shown in FIG. 27, with the conventional measuring device using a planar X-ray detector, a minimum value C1 and a maximum value C2 of detector output values in the interested area C are respectively made to correspond to a minimum value (LSB) and a maximum value (MSB) of an A/D converter which can be subjected to analog-to-digital conversion, so as to make maximum use of the capability of the A/D converter. The thick line 209 shown in FIG. 28 shows the relationship between the range (C1-C2) of the value of the detector output (x-axis) and the range (LSB-MSB) permitting the input (y-axis) to the A/D converter. In the case of the conventional measuring device. thus set, however, as shown in FIG. 25, for example, in the measurement of an interested area A (901a) which includes only a portion close to the heart, i.e., a portion (C1-A1) where pixel values are small, values greater than the maximum value A1 are not inputted to the A/D converter. Similarly, as shown in FIG. 26, in the measurement of an interested area B (901b) in the heart field which includes only a portion (B1-C2) where pixel values are large, values smaller than the minimum value B1 are not inputted to the A/D converter. Hence, effective use is not made of the A/D converter. Thus, with the conventional X-ray based measuring device, there has been a problem in that the detectable range (dynamic range) of the detector is not effectively used in the measurement of areas where the interested areas are small.
The object of the present invention is to provide a technique which makes it possible to enlarge the dynamic range of the measuring device using a planar X-ray detector and permits high-speed imaging, and to provide an X-ray based measuring device which makes it possible to make effective use of the dynamic range of the planar X-ray detector and improve the quality of the image measured. A brief description will be given below of outlines of typical configurations in accordance with the present invention.
(1) An X-ray based measuring device in accordance with the present invention is characterized by comprising: X-ray imaging means which have their detection areas divided into a plurality of detector units, detect X-rays transmitted through an inspection object, and pick up an X-ray image in an interested area of the inspection object; conversion means for converting analog image signals read from the detector units into digital image data under specified conversion conditions for each of the detector units; and re-conversion means for converting the digital image data obtained for each of the detector units under re-conversion conditions corresponding to the specified conversion conditions, wherein the analog image signals are A/D converted into the digital image data under optimum conversion conditions for each of the detector units, and X-ray images in the interested area of the inspection object are sequentially picked up.
(2) The X-ray based measuring device in accordance with the present invention is characterized by comprising: X-ray imaging means which have their detection areas divided into a plurality of detector units, detect X-rays transmitted through an inspection object, and pick up an X-ray image in an interested area of the inspection object; conversion means for converting analog image signals read sequentially from the detector units starting with detecting elements of the detector units which are close to a position where two of the detector units contact with each other into digital image data under specified conversion conditions for each of the detector units; and re-conversion means for converting the digital image data obtained for each of the detector units under re-conversion conditions corresponding to the specified conversion conditions, wherein the analog image signals are converted into the digital image data under optimum conversion conditions for each of the detector units, and X-ray conditions for ensuing imaging are set on the basis of the X-ray image of the inspection object detected by each of the detector units, so as to sequentially pick up X-ray images in the interested area.
Further, the X-ray based measuring devices in (1) and (2) above are also characterized in that (a) setting means is further provided for setting the specified conversion conditions for each of the detector units on the basis of the analog image signals in the interested area set in advance, so as to convert the analog image signals read by each of the detector units into the digital image data; (b) the conversion means has an A/D converter, and a range of an input signal to the A/D converter and a range of a signal detected by the detector unit are made to agree with each other; (c) the conversion means has an A/D converter and linear amplification means for linearly amplifying the signal detected by the detector unit and/or nonlinear amplification means for nonlinearly amplifying the same, so as to change an operating condition of the A/D converter by one of first parameters including a gain and an offset of the linear amplification means and second parameters including a gain and nonlinearity of an input output characteristic of the nonlinear amplification means, or by a combination of the first and second parameters; (d), in (c), the nonlinear amplification means is formed by logarithmic amplification means, and the gain and the nonlinearity are set by a logarithmic slope, an intercept voltage, and an offset voltage; (e) a maximum value and a minimum value in the interested area set in advance are determined for each of the detector units, and X-ray conditions in ensuing imaging are determined on the basis of the maximum value and the minimum value; (f) there are further provided rotating means for rotating about the inspection object X-ray irradiating means for irradiating the inspection object with X-rays and the X-ray imaging means, and reconstructing means for reconstructing an X-ray tomographic image or a three-dimensional reconstructed image of the inspection object on the basis of a plurality of X-ray images picked up; (g), in (f), the rotating means is a means for helically rotating and moving the X-ray irradiating means and the imaging means about the inspection object; and (h) means is provided for changing the interested area set in advance, for each direction in which an image of the inspection object is picked up.
(3) The X-ray based measuring device in accordance with the present invention is characterized by comprising: X-ray imaging means which have their detection areas divided into a plurality of detector units, detect X-rays transmitted through an inspection object, and pick up an X-ray image of the inspection object, wherein analog image signals are read sequentially from the detector units starting with detecting elements of the detector units which are close to a position where two of the detector units contact with each other, and X-ray conditions for ensuing imaging are set on the basis of the X-ray image of the inspection object detected by each of the detector units, so as to sequentially pick up X-ray images in the interested area.
In accordance with the configuration (1), in a case where an X-ray image of the inspection subject is picked up by, for instance, a planar X-ray detector which is apparently formed as a single unit by arranging a plurality of X-ray detector units in an adjoining manner or by a planar X-ray detector whose detection area is regarded as being formed by a plurality of X-ray detector units by controlling the reading of analog image signals, the conversion means first converts analog image signals of the pixels in the interested area into digital image data under optimum conversion conditions most suitable for the X-ray image in the interested area detected (imaged) by each of the X-ray detector units. Next, the re-conversion means re-converts a relationship of correspondence between the analog image signals and the digital image data which is different for each of the X-ray detector units and is based on the specified conversion conditions into a relationship of correspondence between the analog image signals and the digital image data which is set in advance for each of the X-ray detector units. Accordingly, it is possible to effectively use the dynamic range of each X-ray detector unit, and even if A/D converters with limited input capabilities are used, the dynamic range can be expanded as the device without lowering the resolution of the images. In addition, the quality of the X-ray images, i.e., the measured images, can be improved. Further, since reading is effected by splitting one image, high-speed reading becomes possible, thereby permitting high-speed imaging.
In accordance with the configuration (2), in a case where an X-ray image of the inspection subject is picked up by, for instance, a planar X-ray detector which is apparently formed as a single unit by arranging a plurality of X-ray detector units in an adjoining manner or by a planar X-ray detector whose detection area is regarded as being formed by a plurality of X-ray detector units by controlling the reading of analog image signals, analog image signals read sequentially from the detector units starting with detecting elements in the interested area which are closest to a position where two of the detector units contact with each other, and the conversion means converts analog image signals of the pixels in the interested area into digital image data under optimum conversion conditions most suitable for the X-ray image in the interested area detected (imaged) by each of the X-ray detector units. Namely, since the analog image signals outside the interested area are not read, high-speed imaging of the X-ray images becomes possible. Next, the re-conversion means re-converts a relationship of correspondence between the analog image signals and the digital image data which is different for each of the X-ray detector units and is based on the specified conversion conditions into a relationship of correspondence between the analog image signals and the digital image data which is set in advance for each of the X-ray detector units. Accordingly, it is possible to effectively use the dynamic range of each X-ray detector unit, and even if A/D converters with limited input capabilities are used, the dynamic range can be expanded as the device without lowering the resolution of the images.
In accordance with the configuration (3), in the case of the planar X-ray detector formed by a plurality of X-ray detector units, the line connecting the positions where two X-ray detector units contact each other is set in the vicinity of the center of the planar X-ray detector. In addition, in a case where an examiner sets an interested area on the X-ray image, the interested area is frequently set in the vicinity of the center of the X-ray image. When analog image signals are read from the planar X-ray detector, the analog image signals are sequentially read from the pixels in the interested area which are closest to the line connecting the positions where the two X-ray detector units contact each other in the vicinity of the center of the planar X-ray detector. Accordingly, since the analog image signals outside the interested area are not read, high-speed imaging becomes possible.